While about 90 percent of the tropical cyclones affecting Hong Kong occur between June and October, tropical cyclone warnings have been issued in November or even into early December due to the approach of some late-season "autumnal typhoons". Typhoons approaching the South China coast late in the year are often weakened by the northeast monsoon. However, in the early hours of 10 November 1900, Hong Kong was caught off guard by the fierce winds of a typhoon. Since the year 1900 was the "Geng-Zi" year in the Chinese calendar, this late-autumn typhoon event was called the "Geng-Zi typhoon disaster ( 庚子風災 )". Despite being one of the deadliest in Hong Kong history, this typhoon event is less known to many people when compared with the other major pre-World War II typhoon disasters in 1874, 1906, and 1937. Therefore, we have collated information from a number of government reports, newspapers and historical meteorological records published around that time to unveil further details about this autumnal typhoon disaster[1-6].
The ignored typhoon signals
The typhoon originated over the sea to the east of the Philippines on 5 November 1900. It tracked northwestward in the next couple of days and entered the South China Sea on 7 November. It then gradually turned northward edging towards the coast of Guangdong during 8 and 9 November (Figure 1). Based on the weather map at the time and the modern re-analysis, this typhoon appeared to be relatively large in size with a circulation of around 800 to 1000 kilometres in diameter (Figures 2 and 3). As such, both the Manila Observatory and Hong Kong Observatory had given due warnings to the public of the existence of this typhoon over the South China Sea. In Hong Kong, typhoon signals were hoisted starting from 8 November to warn the public of an approaching typhoon . By 6:15 p.m. on 9 November, typhoon gun was fired to warn the public that gale force winds were expected in Hong Kong.
Figure 1 Track of the centre of the "Geng-Zi" Typhoon from 5 to 10 November 1900.
Figure 3 Re-analysis of mean sea level pressure and 10-min Wind on 9 November 1900. (Source : NOAA 20th Century "Reanalysis" ).
With the typhoon approaching, the northeasterly winds in Hong Kong strengthened and reached gale force on the night of 9 November 1900. By 4 a.m. on 10 November, winds reached storm force at the Observatory as the eyewall of the typhoon approached from the south. By 5:00 a.m., the typhoon was at its height in the territory when the hourly mean wind speed at the Observatory reached about 61 knots (113 km/hr) (Figure 4). The center of the typhoon very likely passed over the eastern part of Hong Kong in the following 3 hours as the barometer at the Observatory fell to a minimum of 974.9 hPa at around 6:00 a.m. (Figure 5). This still stands as the lowest sea-level pressure record for November to date.
Figure 4 Hourly mean wind speed and direction at the Hong Kong Observatory Headquarters (HKO Hq) on 9-10 November 1900. The wind speeds were calculated from data recorded by the Beckley anemometer at HKO Hq in 1900 and the conversion factors from HKO Technical Note No. 66.
Figure 5 Hourly mean sea level pressure recorded at HKO Hq from 9 to 10 November 1900.
Given the hourly observations taken at the Observatory through the typhoon event, we have made an attempt to estimate the possible hourly positions as well as the pressure pattern of the typhoon during its passage over Hong Kong. The results are depicted via an animation given here (http://www.hko.gov.hk/img/1900typ_obs_isobar_track.GIF). Note that the isobars shown in the animation were constructed based on the assumptions that the typhoon remained generally circular in shape and its intensity unchanged prior to landfall.
As per the reports from the Governor and newspapers[1, 4], the Observatory had given due notice of this impending typhoon to the public and the inclement weather also fully validated the Observatory's prediction. Regrettably, the public gave less attention to the warnings and little precautions were taken as many of them disbelieved that such a violent storm would affect the territory in this season.
How rare is this November typhoon?
As mentioned earlier, the maximum mean hourly wind speed at the Observatory reached 61 knots at 5 a.m. on 10 November. This value is comparable to the records of some of the well-known Signal No. 10 typhoons, such as Gloria (1957), Ruby (1964) and Rose (1971). In the 130-year record of the Observatory, this Geng-Zi typhoon was the first on record that had made landfall in Hong Kong in the month of November. The second tropical cyclone that had ever made landfall in Hong Kong in November occurred in 1939. This typhoon moved in a rare west to east direction across Hong Kong on 23 November 1939. Signal No. 9 was hoisted during the height of the storm. Although the Hurricane Signal No. 10 did not exist in 1900, and had never been hoisted in November since its inception in 1931, the wind and pressure records at the Observatory strongly suggested that the Geng-Zi typhoon could have brought hurricane force winds in Hong Kong, especially over the eastern part.
Besides the 1900 and 1939 typhoons, there was only one more typhoon (1915) that had brought gale force winds at the Observatory during November before World War II. After World War II, there were two typhoons which necessitated the hoisting of Signal No. 8 or higher in November, namely the typhoons in 1954 (Signal No. 9) and 1972 (Signal No. 8). Coincidently, both of them were named Pamela. Counting all of the above-mentioned typhoons, Signal No. 8 or higher event in November occurred on an average of about every 26 years in the past 130 years in Hong Kong. Thus, statistically speaking, Hong Kong may be already due for the arrival of a late-autumn Signal No. 8 typhoon. The last time that tropical cyclone signal No. 3 or above were hoisted in the month of November due to a typhoon was already back in 1993 during the approach of Typhoon Ira. Although only Signal No. 3 was hoisted for Ira, a brief period of gale force winds was experienced in parts of the territory.
Damages and casualties
The "Geng-Zi" Typhoon caused extensive damages and heavy casualties to the territory (Figure 6). Numerous sampans and boats were sunk or even smashed to matchwood by the raging waves. Ten steam-launches and over 110 junks were also sunk and the harbour was full of wreckage. The gunboat H.M.S Sandpiper sank at her mooring and her crews, with one exception, were gallantly rescued by the torpedo-destroyer H.S.M. Otter. The large dredger "Canton River" was blown over and sunk. The Star Ferry pier was also heavily damaged. Over land, there were many reports of damages to houses, especially in the Peak District. All matsheds erected on the reclamation land over Yaumati were leveled to ground by the high winds. Many trees were damaged or uprooted. Lamp posts and telephone posts were bent at all angles by the furious winds. Over 200 lives were lost in these few fatal hours.
Figure 6 Damages in Hong Kong brought by the "Geng-Zi" Typhoon as reported by the Illustrated London News on 22 December 1900.
Judging from the meteorological records and the historical document descriptions, this typhoon brought fearful havoc and desolation to the ill-prepared society in Hong Kong in 1900. This event highlights that, besides timely and accurate forecasts, the awareness and preparedness of the public to respond are also key factors of an effective typhoon warning system. No matter in peak or off-peak seasons, we should always remain vigilant to the typhoon threat in Hong Kong.
C.M. Shun(1), K.Y. Kong(2), and T.C. Lee(1)
(1) Hong Kong Observatory (2) Weather Prediction Center - NOAA, U.S.A.
 G.S.P. Heywood (1940). "The Typhoon of November 17th to 25th, 1939". Appendix B of the Meteorological Results 1939, Royal Observatory
Friday, 3rd May 2013
Which day is the Tuen Ng Festival for this year (2013)?
Which day in the Gregorian calendar is the first day of the 5th lunar month this year (2013)? This question has aroused public concern and discussion (http://www.ettoday.net/news/20121120/129516.htm). According to the "Hong Kong Observatory (HKO) Almanac 2013" [Figure 1], using the high-precision astronomical data of the HM Nautical Almanac Office, U.K. and the U.S. Navy Observatory, the first day of the 5th lunar month in 2013 will be on 8 June in the Gregorian calendar, same as that of the official almanac of Mainland China. However, some common Chinese folk calendars fix that day to be 9 June. Because of this difference, there appears to be two different Gregorian dates of the Tuen Ng Festival (the 5th day of the 5th lunar month) in 2013.
In fact, such discrepancy is not uncommon. Table 1 shows that there will be three more similar situations in the next 20 years. In particular, there are two different dates for the Lunar New Year Day in 2030!
Table 1 Comparison of Gregorian-Lunar calendar conversion of HKO Almanac and some Chinese folk calendars.
What are the reasons behind the discrepancies? Let us first understand how the first day of a lunar month is determined from the astronomical point of view.
Each month in the lunar calendar starts with a "New Moon", which occurs when the Moon and the Sun move to the same longitude on the ecliptic, . Different computed dates in different calendars come about due to two main reasons: (a) outdated astronomical computation; and (b) change in the standard time adopted.
First of all, astronomical computation has become more and more precise with the advance in science and technology. Official authorities will make use of the latest astronomical data and computation methodology to re-calculate and revise the calendars from time to time. The Hong Kong Observatory and the Zijinshan Astronomical Observatory of Mainland China will revise the official calendars whenever there are updated information and decisions issued by the International Astronomical Union and the Jet Propulsion Laboratory in the US. On the other hand, some of the Chinese folk calendars are compiled based on outdated astronomical data and methodology or duplicating the data from old versions of official calendars. In case when the "New Moon" occurs very close to midnight, it is possible to have different start times for the first day of lunar month based on different astronomical data and computation methodologies. For example, the "New Moon" of the 5th lunar month this year (2013) will occur at 23:56 Hong Kong Standard Time (HKT) on 8 June based on the latest astronomical data and computation methodology. If outdated data and computation methodology are used, the "New Moon" may appear after 00:00 (HKT) on 9 June and the first day of the 5th lunar month will then occur on 9 June. Table 2 shows that all the times for the "New Moons" of the four dates mentioned in Table 1 will occur near midnight based on the latest astronomical data and computation methodology.
Table 2 The Gregorian calendar dates and Hong Kong Standard Times of the New Moons for the first days of the lunar months listed in Table 1.
Secondly, even though the same astronomical data and computation methodology are used, the results may still be different if different standard times are adopted. In late Qing and early Republic period, the 116.383 degree East (longitude of Beijing) of apparent solar time was used as time standard . However, a unified time zone, which is 120 degree East of longitude of apparent solar time, was adopted as the standard time for Mainland China soon after the establishment of the People's Republic of China. The times for "New Moons" and the "Solar Terms" of the calendar was then converted to the Coordinated Universal Time (UTC) plus eight hours. It is noted that the difference in these two standard times adopted may lead to a difference of about 14.468 minutes for defining the beginning of a day. Therefore, adopting different standard times could generate different results in compiling the calendar even though the same astronomical data and computation methodology are used. For example, the first day of the 1st lunar month in 1916 would be on 4 February (the "New Moon" occurred at 00:05 on 4 February) if the current standard time is used, but it was fixed on 3 February (the "New Moon" occurred at 23:51 on 3 February) based on the standard time used at that time.
There are many publications of Chinese folk calendars in the market. Most of them have not specified the authors and source of data. It is therefore rather difficult to trace the source of astronomical data and computation methodology as well as the standard time adopted. As a difference in one minute may cause the difference of one day in Gregorian-Lunar calendar conversion, it is not surprising to find discrepancies between the latest official calendars and folk calendars. Please double-check the Observatory's Almanac if in doubt (http://www.hko.gov.hk/gts/astron2013/almanac2013_index_e.htm) [Figure 2].
Figure 2 The electronic version of the Hong Kong Observatory Almanac.
Monday, 15th April 2013
The Earthquake Monitoring and Information Services of the Hong Kong Observatory
Earthquakes are natural phenomena that are capable of inflicting severe damage, and may induce destructive tsunamis thousands of miles away. The Indian Oceanic tsunami in 2004, the Wenchuan earthquake in 2008, the Haiti earthquake in 2010 as well as the Japanese tsunami in 2011 were all shocking calamities.
Earth tremors are felt in Hong Kong from time to time (twice a year on average), but it has been rather rare for them to bring some minor damages to buildings. Although the chance of destructive earthquakes to occur in Hong Kong is not high (See GEO Information Notes No. 2/2012), as a economic and cultural hub, many facets of Hong Kong would still be affected by overseas earthquakes as well as the associated tsunamis, which will attract the attention of Hong Kong people travelling, planning to travel, and staying abroad as well as that of government organisations and private enterprises. To meet the high demand for information on earthquakes worldwide, the Observatory closely monitors seismic activities and provides timely earthquake information for the public.
The magnitude 7.3 Nan'ao earthquake in 1918, which inflicted minor damage to a few buildings in Hong Kong, propelled the Observatory to start instrumental monitoring of seismic activities with a set of long-period seismographs in 1921. Following the Tangshan earthquake in 1976 which caused great public concern, the Observatory established a local seismograph network with four stations in 1979, thus enabling monitoring and analyses of earthquakes within the territory and around 200 km of Hong Kong. The network was subsequently expanded to 9 stations in 1997. In 2010, a broadband seismograph station near Po Shan Road, Mid-levels on Hong Kong Island was commissioned and became one of the 150+ members of the Global Seismographic Network (Figure 1 & 2). Not only has the Po Shan Seismograph Station significantly enhanced the capability to monitor earthquakes and tsunamis in the South China Sea, it has also become the tenth member of the Hong Kong Seismograph Network (Figure 3). In 2011, real-time accelerometers were installed at five stations in the network, with a view to facilitating the determination of intensities of locally felt earth tremors and more timely issuance of related earthquake reports and messages.
During the past few decades, advances in information technology have greatly expedited the exchange of data and led to more timely seismic analyses. Before the World War II, staff of the Observatory had to manually analyse the arrival time of seismic waves and mailed the results to the International Seismological Summary (ISS), which compiled global earthquake catalogues (Figure 4). The whole process took many days to complete. Since early 1960's, the Observatory started sending analysed seismic messages to the then Pacific Tidal Wave Warning Service and could be able to provide information on larger earthquakes worldwide to local press in several hours after occurrences (Figure 5). In late 1990's, the emergence of the Internet shortened the issuance of earthquake press releases to about one hour. In 2007, the Observatory developed a software package that automated a majority part of the analysis process, further speeding up the issuance to around half an hour. In 2010, the Observatory commissioned the automatic seismic information system, utilising real-time seismic waves collected from seismograph stations all over the world (Figure 6 & 7). In March 2011, the Observatory experimented with the issuance of quick earthquake messages to the public over Twitter (Figure 8), and subsequently extended the dissemination channels to Weibo and RSS for the public, as well as emails and SMS for the electronic media and other special users. The Quick Earthquake Message service became operational in August 2012. Nowadays, quick earthquake messages can typically be issued within 10 minutes after the occurrences of earthquakes of magnitude 6 or above, much shorter than those in the early years that were counted by days.
Figure 1 The Hong Kong Po Shan Seismograph Station near Po Shan Road, Mid-Levels, Hong Kong Island.
Figure 2 The Hong Kong Po Shan Seismograph Station is one of the 150+ stations of the Global Seismographic Network.
Figure 3 Distribution of the ten stations in the Hong Kong Seismograph Network.
Figure 4 Seismic record of the Hong Kong Observatory appearing on International Seismological Summary 1922.
Figure 5 The manuscript of the earthquake press release issued by the Hong Kong Observatory on 27 January 1979.
Figure 6 Observatory's staff conducting seismic re-analyses on computer.
Figure 7 Analyses of the Observatory's Automatic Seismic Information System. The orange circle near Indonesia indicates the location of the epicentre, while the triangles represents those seismograph stations detecting the earthquake.
Figure 8 Quick Earthquake Messages issued by the Hong Kong Observatory through Twitter.